1. Field of the Invention
The present invention relates to magnetic recording, particularly to a magnetic head and disc tester.
2. Description of Prior Art
Prior to description of the invention and for better understanding of terminology used in the specification, it will be helpful to explain the structure of a typical head and disc tester and a testing procedure.
The main purpose of a head and disc tester is to measure the parameters of a magnetic head or a disc; the tester should simulate the motions of the head with respect to the disc which occur in an actual hard-disc drive.
A typical prior-art head and disc tester is shown schematically at 20 in FIG. 1. It has a housing 10 which supports a rotating spindle 12 which in turn supports a hard magnetic disc 14. An arm 16 carries a magnetic head 18 and a drive mechanism (not shown) for moving magnetic head 18 with respect to disc 14. As shown, head 18 and arm 16 are both elongated and have aligned longitudinal axes.
Disc 14 has an outer track OD and an inner track ID; these tracks are not visible on an actual disc since they are invisible magnetic patterns on a magnetic medium. Arm 16 moves head 18 between track ID and track OD.
Depending on the type of a drive unit used, known head and disc testers can be divided into three categories: (1) Testers in which arm 16 has a pivot point which is fixed with respect to the center of disc 14 and a fixed length from the pivot to the magnetic head. Such devices are custom-made, are not adjustable, and are intended only for a specific design. (2) Testers in which the movement of the head with respect to the disc is controlled by a cam. For simulating different types of disc drives, one must replace the cam. (3) Testers in which the head is positioned with respect to the disc by an X-Y manipulator.
In order to understand the difference between these three types of head and disc testers, it is necessary to define "skew angle".
Let us consider, e.g., the case of the tester of category (1), i.e., an non-adjustable tester with a fixed pivot point.
Arm 16 is shown in FIG. 1 as tangent to track ID. In the course of testing the tracks, arm 16 pivots about its pivot point, so that head 18 moves along a curved path from track ID to track OD. The position of head 18 at track OD is shown by broken lines. It can be seen that in this position, arm 16 and the longitudinal axis of head 18 are not tangent to track OD, but rather forms an angle A.sub.OD with respect to the tangent. This angle is called a skew or a tracking error angle. In the example illustrated, skew angle A.sub.ID is zero, but this is shown only for simplicity of explanation; A.sub.ID can have actual values as well as zero.
A disc drive manufacturer usually wants a tester to imitate the head's motions in the same manner as they occur in the disc drive. Actual disc drives usually have definite skew angles A.sub.ID and A.sub.OD, and these skew angles are used as input data for the tester. From this viewpoint, the testers of category (1) are suitable only for fixed skew angles A.sub.ID and A.sub.OD and cannot be used for even slightly changed values of these angles.
The testers of category (2) allow variations of skew angle, but for that purpose a cam must be changed for each variation. This requires that a set of cams be provided. However, the cams permit skew angles to be changed only in discrete steps. Also because the cams are complicated to manufacture and require precision machining, the tester becomes very expensive. Another disadvantage of the testers of category (2) is that their mechanisms have play in their links.
The tester of category (3) can provide a stepless variation, but at a sacrifice of simplicity of the mechanism, as the X-Y manipulator requires separate drives on X and Y axes and the use of complicated and expensive parts and units. Also the testing procedure is time-consuming, as it requires two step motors to move the head in two directions.